1. Field of the Invention
The present invention relates generally to carburetors of the type used with internal combustion engines. More particularly, the invention is directed to an apparatus for matching the air/fuel ratio delivered by a carburetor to the requirements of an engine operating at various speeds and loads.
2. Description of the Prior Art.
Precise air/fuel ratio control, especially on engines operating very lean at part load, is absolutely necessary for good driveability and optimum emissions. When a carburetor is used to mix and meter the air and fuel, it is designed to match as closely as possible the particular requirements of the engine. In addition to a primary fuel metering system which generally comprises an air induction pipe, a venturi throat, a fuel nozzle and a throttle plate, it is not uncommon to employ other systems to tailor the output of the carburetor to the requirements of the engine. For example, choke systems are often incorporated to improve cold starting and running characteristics of the engine, idle systems are used to facilitate low speed engine operation and accelerator pump systems are sometimes used to prevent stalling by injecting an additional quantity of fuel when the throttle plate is rapidly opened.
The primary fuel metering system is a carburetor is usually adjusted to deliver an air/fuel ratio which is appropriate for mid-range engine load conditions. Thus, as engine load increases, the air/fuel ratio provided by the primary fuel metering system must be enriched to obtain the most efficient operating conditions. This required engine enrichment is frequently accomplished with a power jet system which at a predetermined point augments the quantity of fuel being delivered at the venturi throat.
A power jet mixture enrichment system can only approximate an engine's requirements. It cannot proportionately respond to increasing engine loads as it is either on or off and is usually adjusted to meet the engine's requirements under maximum load conditions. Generally, at engine loads greater than normal but less than those required to kick in the power jet, the engine is running lean. At loads sufficiently high to actuate the power jet, but less than full power, the engine runs too rich. Because of these inaccuracies associated with metering the fuel, optimum performance is not obtained over the full range of engine operating conditions. This results in higher fuel consumption and higher exhaust emission levels.
In internal combustion engines equipped with carburetors and turbochargers it is desirable to locate the carburetor between the air compressor discharge and the intake manifold as such placement results in superior mixing of the fuel with the air. However, given the present state of the art, it is extremely difficult to so position the carburetor when high load enrichment is obtained with a power jet system having a bleed air feature because at high loads, the compressor "boost" applied through the air bleed creates a high air flow which leans out the mixture at a time when it should be enriched.